1. Field of the Invention
This invention relates to an image heating apparatus suitable for use as a heating and fixing apparatus carried on a copying machine or a printer using an electrophotographic recording technique or an electrostatic recording technique, and particularly to an image heating apparatus using a roller having an adiabatic layer of porous ceramics.
2. Description of Related Art
In fixing apparatuses in image forming apparatuses adopting an electrophotographic process such as copying machines, printers and facsimile apparatuses, a heating and fixing apparatus is widely, used as a fixing apparatus for fixing an unfixed toner image transferred onto a recording material such as transfer paper or OHP on the recording material. As the heating and fixing apparatus, use has been widely made of one of a heat roller type in which a pressure roller is brought into pressure contact with a heated fixing roller, and an unfixed toner image is heated and fused to thereby fix the unfixed toner image while a recording material is nipped between and conveyed by the two rollers.
FIG. 6 of he accompanying drawings shows a schematic view of a heating and fixing apparatus of the heat roller fixing type. A fixing roller 5-1 is of structure in which a heat source 5-2 such as a halogen lamp is contained in a metal core 5-1a made of a metal, and an elastic layer 5-1b formed of silicone rubber or the like and a releasing layer 5-1c of fluorine resin are provided on the outer peripheral surface of the metal core. A pressure roller 5-3 brought into pressure contact with the fixing roller 5-1, like the fixing roller, has an elastic layer 5-3b and a releasing layer 5-3c formed on the outer peripheral surface of a metal core 5-3a. 
The entire fixing roller 5-1 is warmed by the heat source 5-2 contained in the interior of the fixing roller 5-1. Also, part of the energy is transmitted to the pressure roller 5-3, which is also warmed. When a transferring material 5-5 as an image recording medium bearing an unfixed toner image 5-4 thereon passes through a nip N5 between the fixing roller 5-1 and the pressure roller 5-3, the unfixed toner image 5-4 is heated and fused by the contact heat transfer between the fixing roller 5-1 and the pressure roller 5-3, and is fixed on the transferring material 5-5.
In the heating and fixing apparatus of the heat roller type, however, the heat capacity of the metal core 5-1a of the fixing roller 5-1 is great and therefore, the time until the nip N5 is raised to a predetermined fixing temperature becomes long. Nowadays when energy saving is demanded, there is required a fixing apparatus which is good in efficiency and quick in rising. FIG. 7 of the accompanying drawings shows the consumed power waveform of the heating and fixing apparatus of the heat roller fixing type with time as the axis of abscissas and power as the axis of ordinates. The shown power waveform A is the result of the measurement of power consumed by the heating and fixing apparatus from the moment when the power supply switch of a printer has been closed until the termination of the continuous printing of 200 sheets. According to a graph line in FIG. 7, the consumed power has lowered to the order of 500 W after power of 700 W has been supplied for about 180 sec. First, the section in which constant power of 700 W is consumed is the section of rising of the fixing apparatus. Power is consumed at full power in order to heat the fixing apparatus to a predetermined fixing temperature, and the power waveform exhibits constant power. When the fixing apparatus is raised to the predetermined fixing temperature, the conveyance of paper is started and printing is started. This start of the printing is indicated by the timing at which the consumed power indicated by the power waveform A changes from 700 W to 500 W. This is because control (temperature control) for maintaining a predetermined temperature has begun. During the printing, the power is consumed to make up for chiefly the radiation of the fixing apparatus, heat taken away by the printing paper when it passes through the fixing apparatus, and heat given to a toner, and usually, power consumed during the printing is low as compared with power supplied at full power during the rising of the fixing apparatus. Accordingly, it is possible to read the rising time of the fixing apparatus from the waveform of the consumed power.
In the description of the present invention, consideration about the rising time will be described with some power waveforms shown, and on the basis of the above-described mechanism, discussion will be made with the time required until a point at which the power consumed at full power changes and becomes low as the rising time.
The rising time is shortened, whereby the first print-out time can be shortened, and this in turn leads to a reduction in the consumed power. To shorten the rising time, the heat capacity of the fixing roller can be made small, and as one of countermeasures, there has been studied a method of making the wall thickness of the metal core of the fixing roller small to thereby make the heat capacity thereof small. However, when the wall thickness of the fixing roller is made small, the mechanical strength of the roller becomes weak and the fixing roller becomes curved at the nip portion whereat it contacts with the pressure roller and the contact pressure of the central portion thereof becomes weak, and the nip is decreased and the fixing intensity is reduced. In order to prevent this inconvenience, various methods of reinforcing the fixing roller have been proposed.
For example, Japanese Patent Application Laid-open No. S59-155875 proposes a fixing roller having radial ribs provided horizontally with respect to the axis of the roller on the inner surface thereof. Also, Japanese Patent Application Laid-open No. H11-149226 proposes a fixing roller having internal structure in which radial ribs are inclined and extended with respect to the axis of the roller.
FIG. 8 of the accompanying drawings shows a schematic view of the fixing roller having radial ribs provided horizontally with respect to the axis of the roller on the inner surface thereof. An elastic layer 6-2 formed of silicone rubber or the like and a releasing layer 6-3 formed of fluorine resin are provided on the outer peripheral surface of a metal core 6-1 provided with the ribs. The metal core 6-1 is consolidated by the ribs on the inner surface, and can keep its strength even if the wall thickness thereof is made small.
By these propositions, the wall thickness of the fixing roller is made small and the shortening of the rising time of the fixing apparatus is done while the strength of the roller is kept and the fixing intensity is kept.
There has also been proposed a heat roller fixing apparatus of an external heating type in which a heat source so far contained in the interior of a fixing roller is disposed externally of the roller.
Japanese Patent Application Laid-open No. 2002-40855 proposes a heat roller fixing apparatus having an external heating apparatus and using a material having an adiabatic property for a pressure roller. Japanese Patent Application Laid-open No. 2002-221219 proposes a heat roller fixing apparatus having an external heating apparatus and using a material having an adiabatic property for a fixing roller.
In the respective constructions, it becomes possible to quickly warm the surface of the fixing roller by the external heating apparatus, and it becomes possible to shorten the rising time of the fixing apparatus.
Also, in these propositions, one of the pressure roller and the fixing roller is formed of a material excellent in adiabatic property, and design is made such that the rising time can be further shortened.
In the construction of Japanese Patent Application Laid-open No. 2002-40855, the pressure roller is formed of a material of high hardness excellent in adiabatic property, and the fixing roller opposed thereto is of structure in which an elastic layer is provided on a metal core. As the material excellent in adiabatic property, use is made of porous ceramics of high hardness, and even if pressure is applied thereto, pores are not crushed, but it is possible to maintain the adiabatic property. The fixing roller opposed to the pressure roller is also provided with an elastic layer to thereby secure a fixing nip. In this construction, design is made such that during the rising, it is difficult for the heat of the fixing roller to be taken away by the adiabatic pressure roller and a rising speed is heightened.
On the other hand, in the construction of Japanese Patent Application Laid-open No. 2002-221219, the fixing roller is formed of a highly adiabatic material, and the pressure roller opposed thereto is of structure in which an elastic layer is provided on a metal core. As the material excellent in adiabatic property, use is made of porous ceramics of high hardness, and even if pressure is applied thereto, pores are not crushed, but it is possible to maintain the adiabatic property. Also, the pressure roller opposed to the fixing roller is provided with an elastic layer to thereby secure a fixing nip. In this construction, the effect that only the surface layer of the adiabatic fixing roller can be quickly raised in temperature heightens, and design is made such that the rising speed is heightened.
FIG. 9 of the accompanying drawings shows a schematic view of a heating and fixing apparatus which is provided with an external heating apparatus in which one roller is formed of a material excellent in adiabatic property. A fixing roller 7-1 is of structure in which an elastic layer 7-1b and a releasing layer 7-1c are provided on the outer peripheral surface of a metal core 7-1a made of a metal. A pressure roller 7-2 brought into pressure contact with the fixing roller 7-1 has an adiabatic material layer 7-2b formed of porous ceramics or the like and a releasing layer 7-2c formed externally of a metal core 7-2a. Heating means 7-3 having structure in which a heater 7-3b is provided in a roller 7-3a made of a metal abuts against the external portion of the fixing roller 7-1, and the fixing roller 7-1 is heated by this heating means 7-3, and the fixing roller performs a fixing operation after the surface temperature thereof has reached a fixing temperature. During the rising of the fixing apparatus, only the vicinity of the surface of the fixing roller 7-1 is warmed and therefore, it is possible to quickly raise the surface temperature of the fixing roller 7-1.
Further, the pressure roller 7-2 is heat-insulated and therefore, during the rising, it is difficult for the surface heat of the fixing roller 7-1 to be taken away by the pressure roller 7-2, and it is possible to raise the surface temperature of the fixing roller more efficiently in a case where for example, the both rollers are constituted by elastic layers formed of rubber or the like.
FIG. 10 of the accompanying drawings shows a graph of power waveforms in a conventional heat roller fixing type, a heating roller fixing type using a fixing roller on the inner surface of which radial ribs are provided horizontally with respect to the axis of the roller, and a heating roller fixing type provided with an external heating apparatus, with time as the axis of abscissas and power as the axis of ordinates. These power waveforms have been measured under a process condition in which the fixing intensity of an unfixed toner image onto a recording material becomes the same at a conveying speed of 200 mm/sec for the recording material.
The fixing intensity represents with how much force an unfixed image fixed by the fixing apparatus is fixed on the recording material, and is represented by a density reduction rate (unit: %). Description will now be made of a method of measuring the density reduction rate.
As the unfixed image, use is made of black and halftone (gray) images of 5 mm square disposed at nine locations on letter-size paper.
The halftone pattern of the unfixed image is a pattern in which pixel density of 600 dpi was formed by a matrix of 3×3 and this was formed in a staggered shape with one dot one space.
The density of the halftone of the image after passed through the fixing apparatus is measured by a density measuring machine (produced by Macbeth Co., Inc.), whereafter the image is rubbed by a rubbing test machine for exclusive use, and the density of the halftone after rubbing is again measured, and the reduction rate of the density is calculated.
The rubbing test machine is of structure in which metallic weight of 200 g is placed on a stand for fixing paper thereon by electricity in accordance with the black and halftone patterns of 5 mm square disposed at nine locations on the paper. Silbon C paper (produced by Ozu Corporation) is sandwiched between the paper and the weight. The stand for fixing the paper thereon is adapted to be reciprocally movable in the longitudinal direction of the paper, and at this time, the image is rubbed by the Silbon C paper and is broken. In the present embodiment, the image was rubbed by the stand reciprocally moved five times.
This reduction rate of the density is calculated with respect to all of the halftone images at nine locations on the letter-size paper, to thereby calculate the average value, and is used as an index representative of the fixing intensity under that condition.
In the measurement at this time, the process condition in each fixing process was defined so that in a laboratory kept at humidity of 50%, the density reduction rate in rough paper (Fox River Bond produced by Fox River Paper Co.) of basis weight of 90 g as a recording material might be 10%.
If under the above-described environment, the density reduction rate is 10%, it is usually such a level that even if the toner is strongly rubbed by fingers, the toner will not come off from the paper, and is at a level which can be sufficiently fit for practical use.
A power waveform A in the heating roller fixing type in FIG. 10 is the same as that plotted in FIG. 9.
During the rising of the fixing apparatus, power of 700 W is consumed, whereafter temperature control is started and consumed power lowers to 500 W. The time from the start of electrical energization until the consumed power lowers is 180 sec., and the time required for the rising can be 180 sec.
Likewise observing the graph, the rising time is greatly shortened in the heating roller fixing type using a fixing roller provided with ribs on the inner surface thereof and made small in the wall thickness thereof, and the heating roller fixing type provided with an external heating apparatus.
In FIG. 10, a power waveform indicated by B is a power waveform in the heating roller fixing type using a fixing roller provided with ribs horizontal with respect to the axis of the roller. According to this, the time until the power so far consumed by 700 W lowers to 500 W is about 60 sec. A power waveform indicated by C is a power waveform in the heating roller fixing type provided with an external heating apparatus, and in this fixing type, a change is seen in the power in 40 sec. Accordingly, in the heating roller type using the fixing roller provided with the ribs and made small in the wall thickness thereof, the rising time can be considered to be 60 sec., and in the heating roller type provided with the external heating apparatus, the rising time can be considered to be 40 sec., and it is seen that as compared with the conventional heating roller type, both of these greatly shorten the rising time.
The rising time has been shortened because of the curtailment of the heat capacity by the smaller wall thickness of the roller, and a construction in which the surface of the fixing roller is quickly warmed by the external heating apparatus and further, it is difficult for the heat to be taken away from the surface of the warmed fixing roller by the pressure roller.
By the curtailment of the heat capacity and an improvement in the heating method, the shortening of the rising time has been advanced also in the heat roller fixing type.
An improvement in the heating and fixing apparatus of the heating roller fixing type has been advanced, while on the other hand, there has been proposed an example of a heating and fixing apparatus using a method which eliminates the necessity of supplying power to the heating and fixing apparatus during standby and which minimizes consumed power, and more particularly a film heating type in which a toner image on a recording material is fixed through thin film of small heat capacity interposed between a heater portion and a pressure roller (for example, Japanese Patent Application Laid-open No. H2-157878 or Japanese Patent Application Laid-open No. H4-44075).
FIG. 11 of the accompanying drawings shows a schematic view of the fixing apparatus adopting the film heating type. This fixing apparatus comprises a ceramic heater 9-1 as a heating member, a stay 9-2 which is a supporting member adiabatically supporting the heater 9-1, fixing film 9-3 of a thin-walled cylindrical shape formed of a heat-resistant resin material and twined on the stay 9-2 supporting the heater 9-1, a pressure roller 9-4 brought into pressure contact with the heater 9-1 with the fixing film 9-3 interposed therebetween to thereby form a nip portion N9, etc.
The rotative driving of the pressure roller 9-4 is done, and along therewith, the fixing film 9-3 is driven to rotate, the heater 9-1 is electrically energized and in a state in which it is temperature-controlled to a predetermined temperature, a recording material 9-6 bearing an unfixed toner image 9-5 thereon is conveyed to the nip portion N9, and is nipped by and conveyed through the nip portion N9 together with the fixing film 9-3 to thereby impart the heat of the heater 9-1 to the recording material 9-6 through the fixing film 9-3 and fix the unfixed toner image 9-5.
The fixing film 9-3 is thin and small in heat capacity and good in heat responsiveness and therefore, the time required from after the heater 9-1 is electrically energized until it is temperature-controlled to the predetermined temperature is short, and energy saving accompanying this is realized.
FIG. 12 of the accompanying drawings shows a result obtained by plotting the waveform of power consumed in each heating type by the heating and fixing apparatus from a moment when the power supply switch of a printer has been closed until the termination of the continuous printing of 200 sheets, with time as the axis of abscissas and power as the axis of ordinates. These power waveforms have been measured under a process condition in which at a conveying speed of 200 mm/sec. for the recording material, the fixing intensity of the unfixed toner image onto the recording material becomes the same. Power waveforms indicated by A to C are by three fixing types, i.e., the heat roller type and the heating roller types by the fixing roller provided with the ribs on the inner surface thereof, and these are the same as the plots shown in FIGS. 7 and 10. In FIG. 12, according to the waveform A, in the heat roller type, when power of 700 W is supplied to the heater, about 180 sec. is required until the fixing apparatus completely rises. According to the waveform B, in the heating roller type using the fixing roller provided with the ribs on the inner surface thereof to thereby make the wall thickness thereof, the heat capacity of the roller was reduced and therefore, the rising time of the fixing apparatus is shortened to 60 sec. According to the waveform C, the external heating apparatus is provided and the surface temperature of the fixing roller is quickly raised, whereby the rising time can be shortened, and the rising time is shortened to 40 sec. In the film heating type represented by D, a member of smaller heat capacity is used. The power consumed by 700 W at the early stage of the start of electrical energization immediately lowers to 500 W, and the time hitherto required is about 10 sec. Accordingly, the time required for rising is 10 sec. and as compared with the other heating and fixing types, very quick rising of the fixing apparatus is realized.
As described above, the time until the entire fixing apparatus is warmed and temperature control is started has been shortened and energy saving has been done.
In the heat roller fixing type, by a construction in which the curtailment of the heat capacity is done by the thin-walled roller provided with the rib structure on the inner surface thereof and the rising time becomes shorter, and the vicinity of the surface of the fixing roller is quickly warmed by the external heating apparatus and the adiabatic property of the pressure roller opposed to the fixing roller is enhanced to thereby make it difficult for the heat to be taken away, the shortening of the rising time has been advanced. Also, in the film heating type, film of small heat capacity is adopted, whereby the further shortening of the rising time has been done.
However, as shown also in FIG. 12, in any of the heating roller fixing type, the heating roller fixing type using the thin-walled fixing roller, the heating roller type having the external heating apparatus and the film heating type, there is no great change in the consumed power during the passing of the recording material, and substantially equal power is consumed. As a factor which fixes the toner, the effect by heat transfer is dominant, and during the passing of paper, the movement of heat is effected through the nip between the upper and lower rollers. Also, during the passing of paper, fixing depending on heat transfer is dominant and therefore, in any fixing type, the average consumed power during the printing after the start of temperature control has been substantially equal (about 500 W).
In the heating and fixing type, heat is imparted to an unfixed toner image on a recording material in the fixing nip by contact heat transfer to thereby effect fixing, and during the passage of the recording material, much of the heat in the nip is taken away by the recording material.
To obtain the same fixing intensity by the use of the same toner and at the same recording material conveying speed, in any type, it is necessary to supply the same amount of heat into the nip, and the amount of heat taken away during the passage of the recording material also becomes substantially equal.
Accordingly, the power consumed to make up for it becomes substantially equal in any type.